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Old 15th January 2010, 11:19 AM   #1
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Default Mystery resistor in simple audio amp

Gentlemen,

This circuit was published in the "FET Cookbook" (Das FET Kochbuch) written by the Texas Instruments application guys, back in the seventies (as far as I know it was not published in English). The question I have is not about the audio qualities of the design but the function of the 4.7 KOhm resistor connected between the drain of the FET (T1) and the base of the BC212 driver (I marked it as R2).

Apart from the possibility of eliminating a large surplus of 4.7 K resistors in the lab, I really honestly cannot see any reason for including it in the circuit but somehow I cannot imagine that the guys at TI would have put it in without a good reason, so I was wondering if you have any ideas.

At the 200 uA-ish drain current the FET is operating at, its output impedance is very high (above 100 kOhm) so the 4.7 K in series hardly makes any difference. I can't see it would make any difference from a DC biasing point of view either, but then what is it for?

I must confess I'm baffled. The rest of the circuit makes sense.

Thanks for any suggestions.
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Old 15th January 2010, 02:27 PM   #2
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Quote:
Originally Posted by MajorMajor View Post
Gentlemen,

This circuit was published in the "FET Cookbook" (Das FET Kochbuch) written by the Texas Instruments application guys, back in the seventies (as far as I know it was not published in English). The question I have is not about the audio qualities of the design but the function of the 4.7 KOhm resistor connected between the drain of the FET (T1) and the base of the BC212 driver (I marked it as R2).

Apart from the possibility of eliminating a large surplus of 4.7 K resistors in the lab, I really honestly cannot see any reason for including it in the circuit but somehow I cannot imagine that the guys at TI would have put it in without a good reason, so I was wondering if you have any ideas.

At the 200 uA-ish drain current the FET is operating at, its output impedance is very high (above 100 kOhm) so the 4.7 K in series hardly makes any difference. I can't see it would make any difference from a DC biasing point of view either, but then what is it for?

I must confess I'm baffled. The rest of the circuit makes sense.

Thanks for any suggestions.
Yes it is funny. The only think that comes to my mind right now is that it may have a function in overload prevention or overdrive recovery. What's the supply voltage? It may limit the max current in the FET at overdrive.

jd
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Old 15th January 2010, 06:10 PM   #3
Elvee is offline Elvee  Belgium
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It could be a way of controlling possible VHF oscillations: T1 sees pretty unpredictable HF impedances on its gate and source, and also connecting the drain to such an impedance might be enough to complete an oscillator circuit.
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Old 15th January 2010, 06:21 PM   #4
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The resistor will limit the current as the 50 uF cap charges up.
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Old 15th January 2010, 06:31 PM   #5
teemuk is offline teemuk  Finland
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As far as I know, that resistor is decreasing open-loop gain but I don't really see the point for that.
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Old 15th January 2010, 06:38 PM   #6
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Originally Posted by sawreyrw View Post
The resistor will limit the current as the 50 uF cap charges up.
Yes that seems a good reason for it.

jd
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Old 15th January 2010, 06:39 PM   #7
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Originally Posted by teemuk View Post
As far as I know, that resistor is decreasing open-loop gain but I don't really see the point for that.
I don't think it (R2) does anything on the gain.

jd
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Old 15th January 2010, 08:12 PM   #8
wahab is offline wahab  Algeria
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without R2, the fet drain would be connected to
positive rail through the base emitter junction
of T2 with no limit in the possible drained AC current
other than the 47R resistor in serial with the source
50 uF capacitance...
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Old 15th January 2010, 09:34 PM   #9
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Quote:
Originally Posted by MajorMajor View Post
Gentlemen,

This circuit was published in the "FET Cookbook" (Das FET Kochbuch) written by the Texas Instruments application guys, back in the seventies (as far as I know it was not published in English). The question I have is not about the audio qualities of the design but the function of the 4.7 KOhm resistor connected between the drain of the FET (T1) and the base of the BC212 driver (I marked it as R2).

Apart from the possibility of eliminating a large surplus of 4.7 K resistors in the lab, I really honestly cannot see any reason for including it in the circuit but somehow I cannot imagine that the guys at TI would have put it in without a good reason, so I was wondering if you have any ideas.

At the 200 uA-ish drain current the FET is operating at, its output impedance is very high (above 100 kOhm) so the 4.7 K in series hardly makes any difference. I can't see it would make any difference from a DC biasing point of view either, but then what is it for?

I must confess I'm baffled. The rest of the circuit makes sense.

Thanks for any suggestions.
This resistor (by your circuit in the drain line) I must often introduce in amps, that I have for service to reduce the C-E (D-S) voltage (and loss power) about the first gain stage transistor (if there are differential amp then two resistor must be introduce).

Especially in such cases, if the whole amplifier runs with supply voltages above 100 volts. NAD's integrated amplifier 3240PE is a good example - please look to the PCB hot spot (black PCB) arround the LTP (2x 2SC2240 differential input amp). After introduce the appropriate resistors in the collector lines the thermal loss is much lower (only 20 degrees above ambient temperatur).
An additional advantage is now the use of normal 30V small signal types like BC559C (or BC549C - according the circuit) independend of the individual used voltage supply.

Last edited by tiefbassuebertr; 15th January 2010 at 09:38 PM.
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Old 15th January 2010, 09:51 PM   #10
mjf is offline mjf  Austria
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hello.
i think R2 is a protection res.
the bf 245a has only an Idss around 2mA.
and at the turn on moment some transistors are fully conducting........
greetings..............
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